In rocketry it is necessary to store the fuel for the rocket during a stand-by condition of the rocket. As an example, a rocket may have two storage tanks, for liquid fuel and liquid oxidant respectively, in which the liquid is contained within a bladder within the tank. The space (ullage) between the tank wall and the bladder is under a vacuum, so that its pressure may be approximately 100 microns. The stand-by condition may continue for ten years or more, and it is necessary to monitor the integrity of the ullage to be sure that no air leaks from outside the tank into the space, and that no liquid leaks from within the bladder into the space. If the pressure sensed in the space becomes greater than a selected value, say 100 microns, the sensor must give an alarm to a monitor circuit.
When the rocket is made operative, the liquid is forced to the engine from the tank by pressurizing the ullage up to 315 PSIA. While the instrument is no longer required to operate, it must nevertheless survive being pressurized to 350 PSI without exploding or fracturing, to release the fuel-feeding pressure or to damage the tank or other near-by equipment.
The needs of the application are such as to dictate structure either of stainless steel or of aluminum, and the structure must be an all welded design with a low leakage rate on the order of 1.times.10-8 standard cubic centimeters per second at 100 PSI helium differential.